Loop detectors function by having a circuit detect the change in inductance of a wire loop when a vehicle enters the vicinity of the loop. For instance, in some previous systems, the loop inductance changes by approximately four percent when a vehicle enters the loop. This change of inductance may be detected by sampling the change of frequency of electrical signals used to oscillate the loop.
Loop detectors are frequently used at traffic lights to indicate that a vehicle is present at an intersection and to initiate the changing of the light so that the vehicle can proceed through the intersection. In another example, loop detectors are sometimes placed in front of a barrier, such as a gate or a garage door. When a vehicle enters the loop, the device indicates the detection of the vehicle to a moveable barrier operator system and an action may be taken. For instance, the door or gate may be opened or closed.
As mentioned, loop detectors may be used in conjunction with barrier movement operators. Barrier movement operators are automated systems which are used to move a barrier with respect to an opening. Examples of barriers to be moved include garage doors, gates, fire doors and rolling shutters. A number of barrier movement operators have been sold over the years most of which include a head unit containing a motor connected to a transmission. The transmission, which may include, for example, a belt drive, a chain drive, a screw drive, gear drive or extendible arm is then coupled to the barrier for opening and closing the barrier.
The physical and electrical characteristics of the loop detector circuit drift with temperature and other environmental conditions. Consequently, the detection threshold becomes unreliable and must be changed if accurate responses to loop inductances are to be made. Previous loop detector systems compensated for the component drift by using a running average filter in the loop detector. However, the running average filter approach has proven inadequate to compensate for long range changes in system operation. For instance, the running averages approach often causes the loop detector to ignore a vehicle when the vehicle was on the loop for a long period of time or when a number of vehicles traversed the loop sequentially. Consequently, the gate might never be opened to allow a waiting vehicle to pass through the gate. In other circumstances, the gate might close prematurely damaging one of the string of vehicles passing through the gate.